Optical modules such as optical modulators have been focusing on high-speed modulation rates and large-scale configuration in accordance with an increase in transmission capacity of recent optical transmission systems. Such an optical module, when installed in an optical transmitter, is desirably smaller in size by integrating a plurality of Mach-Zehnder configurations forming an optical waveguide on one chip. The optical module includes, for example, four Mach-Zehnder configurations that form parallel optical waveguide arms. On each pair of optical waveguide arms, two signal electrodes and two ground electrodes are patterned. The optical module inputs different electric signals to the two signal electrodes to generate multi-level modulation signals. The optical module having such a configuration includes input parts to which electric signals are input, and all the input parts are disposed at one side of a package to facilitate mounting and to minimize their footprint.
Such an optical module having input parts disposed on one side of a package receives electric signals such as radio frequency (RF) signals via coaxial connectors provided on a side wall of the package. The coaxial connectors are connected with coaxial adopters through which external electric signals are input. With this configuration, the optical module needs a wider interval between the signal electrodes to which electric signals are input to allow the coaxial adopters having a certain width to fit in. When the number of channels is increased, the optical module needs a larger footprint for the input parts.
To reduce such footprint, a surface-mounting optical module has been developed that receives electric signals from a printed circuit board (PCB) via a flexible printed circuit (FPC) provided to the package. In such an optical module, the FPC, an end of which is soldered to the PCB, extends along the longitudinal direction of the package, and is electrically connected with signal electrodes on an optical waveguide via a relay substrate accommodated in the package to input electric signals. In the optical module, signal line patterns on the relay substrate are electrically connected with the FPC via lead pins of glass terminals embedded in the package. Electric signals such as RF signals input from the PCB to the FPC first reach the lead pins via the FPC and then are input to the signal line patterns on the relay substrate. Passing through the signal line patterns on the relay substrate, the electric signals reach the signal electrodes on the optical waveguide. This configuration eliminates the coaxial adopters and reduces the interval between the signal electrodes to which electric signals are input, whereby the footprint of the input parts can be reduced. Consequently, the optical transmitter can be reduced in size. Conventional technologies are described in Japanese Laid-open Patent Publication No. 2012-48121, for example.
A gap is formed between the lead pins of the glass terminals embedded in the package and a side wall of the package. When the distance across the gap between the lead pins and the side wall of the package corresponds to a certain distance, resonance occurs between the lead pins and the side wall of the package. In particular, an optical module such as an optical modulator dealing with high-frequency signals suffers disturbance of electric field distribution of high-frequency signals caused by the resonance between the lead pins and the side wall of the package described above, resulting in degradation of the high-frequency characteristics.